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Re: [TCML] VTTC New Grid Coil Geometry

Greetings John
The pancake grid coil, on initial experiments, seems to be a very stable arrangement. I did find that I could reduce the number of turns on the pancake so that the variac could be cranked up higher before the voltage was sufficient to oscillate. This is a minor advantage as you can run up the plate voltage without hitting the grid with too much voltage. As with all VTTC tuning was critical. Simply using the reduced size toroid improved performance greatly. I won't know exactly how improved, until I get the thing on a lower bench, but when you have been around these VTTCs it isn't hard to see improvement. However, the sound and spark quality was consistent with the standard arrangement. I don't need a staccato controller yet and digital pictures verify the sword like sparks. What is amazing are the bright white power arcs and I am not sure yet how long they get. This coil has no metal except for wires & toroid and is five feet away from the power supply and oscillator, however, the tank capacitor is adjacent to the coil with short leads.
It keeps the coil away from me ( reduce RF burns) and other surrounding elements that act as stray capacitance or antennas. My next run will use an HV twisted pair for the grid leak leads. There is much yet to learn, especially understanding the coupling Physics of the three coils.
One thing, you are right on. It looks really neat and clean with the hidden grid coil and flash over is eliminated.
Incidentally, thanks for spinning all the different size toroids. Due to your efforts I have accumulated a set that stacks like a toroid christmas tree. 
Regards
John W. G.

John W. Gudenas, Ph.D.
Professor of Computer Science

On Sep 17, 2008, at 4:31 PM, futuret@xxxxxxx wrote:


Dr. John,
I used a still different grid coil arrangement on my 36" spark Tesla coils. Both used flattish primaries. The grid coils were very close to the secondary and were solenoid coils only 1" high and about 1/2" wider diameter than the secondary. The grid coil was in the same plane as the primary (in a 
sense).  The
grid coil consisted of two layers of winding, about 19 turns total of
18awg pvc insulated wire.  I raised or lowered the grid coil to adjust
things.  These coils were very tricky to adjust, and they never gave
swordlike sparks, they always gave fuzzy sparks.  I don't know if the
arrangement offered any real advantage or disadvantage overall.

Then on my TT-27 VTTC, I attempted to use a flat primary and
put the grid coil along the outside of the primary.  This worked
very poorly.

I like your idea of a hidden grid coil.

Cheers,
John
-----------


-----Original Message-----
From: Dr. John W. Gudenas <comsciprof@xxxxxxxxxxxxx>
To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
Sent: Wed, 17 Sep 2008 12:01 am
Subject: Re: [TCML] VTTC New Grid Coil Geometry
Over the past two months I have been experimenting with a different grid coil geometry. On my dual 833A coil I fabricated a flat pancake coil out of 14 awg wire for grid feed back. The inner diameter is approximately the same as the primary solenoid. 
It has 24 turns and is placed 1" under the lowes
t secondary winding. The whole arrangement is covered by a phenolic disc and presents the appearance that the grid coil doesn't exist.
With this geometry I have changed the coupling association that exists among the three coils. I haven't as yet determined the exact relationships, albeit it works great. The coil required some tuning changes that were accomplished by reducing the secondary self capacitance with a smaller toriod and minor adjustment of the grid leak resistor.
I use two identical plate transformers 2800 volts @ 280 ma in parallel through a doubler. I need to lower the coil as it power arcs around 24" to the floor joists above. I suspect continuous 30" corona at full variac. I have no time now to work on it as I ended up on the University Personnel Committee deciding tenure issues. As far as I know this is a different approach to grid coils. Flash over is completely eliminated and I suspect (but have not proved) there is greater magnetic coupling to the primary. Bert Hickman was over a few weeks ago and saw the prototype. When things settle down I'll put up some pictures. You have. I believe, a new alternative now. Let me see what you folks can do with it. 
Regards
John W. G.

John W. Gudenas, Ph.D.
Professor of Computer Science

On Sep 16, 2008, at 9:14 PM, S&JY wrote:


SNIP+++++++++++++++++++SNIP





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